- Enrolled patients will be ventilated by either AVEA (CareFusion, Yorba Linda, CA, USA),e500 (Newport Medical Instrument Inc. CA, USA) or Eivta 4 (Drager) ventilator. When evaluating Raw, the ventilator settings will be transiently switched to volume control mode with fixed tidal volume (500 ml) and constant flow (with a rate of 60 L/min). Plateau pressure is measured by manually controlled end-inspiratory pause12. Every effort will be exercised to avoid excessive airway secretion or patient agitation during measurement. Each measurement will be repeated three times with an interval of at lease 1 minute and calculate their average. Raw will be calculated by an equation of (peak inspiratory pressure - plateau pressure)/flow. After each measurement, the ventilator will soon be reverted to its usual settings. Raw will be routinely determined every 8 hours for 28 days if the ventilator is not discontinued.